A desalination device using a reverse osmosis membrane (hereinafter referred to as an RO membrane) is operated by using reverse osmotic pressure, and a plurality of serially connected RO membrane elements 222 are accommodated in a pressure vessel 224 having a cylindrical configuration as shown in FIG. 16. The RO membrane elements 222 are connected by their water collection pipe 234 at the center of each RO membrane element 222. The feedwater is fed to the desalination device from one end by a high pressure pump to pressurize the interior of the pressure vessel 224 by controlling the opening degree of the valve provided on the concentrated water side. When the applied pressure exceeds the osmotic pressure of the feedwater, the feed water permeates through the RO membrane and the desalinated water (permeated water) flows into the water collection pipe 234 provided at the center of the element.
When the water is supplied to the pressure vessel, the salt concentration increases from the feedwater side to the concentrated water side, and the pressure in the pressure vessel 224 is finally determined by the salt concentration at the final stage, amount of the permeated water, and flow rate of the feedwater on the membrane. Accordingly, excessive pressure is applied in the pressure vessel 224 on the feedwater side and amount of the permeated water increases on this side. For example, FIG. 17 shows relative flux in relation to the position of the RO membrane element with seven RO membrane elements 222 serially arranged. The position of the element in FIG. 17 is the number of elements counted from the feed water side. As shown in FIG. 17, a larger amount of water permeates through the RO membrane on the feedwater side, and amount of the permeated water decreases toward the concentrated water side. Since salt concentration of the water treated in the element increases toward the concentrated water side, a higher pressure is required on the concentrated water side while equal pressure is applied on the feedwater side. Because of such unevenness of the amount of the permeated water in the pressure vessel 224 as shown in FIG. 17, a higher power is required and fouling of the RO membrane element proceeds in the feedwater side.
With regard to the fouling of the RO membrane element, upstream side of the element where the untreated water is supplied tends to suffer from organic and microbial fouling (biofouling) while downstream side of the element from which concentrated water is discharged tends to suffer from scaling by inorganic substance. More specifically, the organic substances and microorganisms in the untreated water tend to be deposited on the RO membrane immediately after the introduction of the untreated water to the element while precipitation of the inorganic substance, that is, deposition of the contaminant inorganic substance on the RO membrane tends to occur on the RO membrane in the downstream side of the element where water treatment has proceeded to some extent and concentration of the water has increased.
In order to remove such fouling of the RO membrane, for example, Patent Literature 1 discloses heating of the reagent solution used for cleaning the reverse osmosis membrane blocked by organic polymer substance. Patent Literature 2 discloses a device which monitors performance of the reverse osmosis membrane and operates a cleaning unit when detecting fouling of the membrane. Patent Literature 3 discloses an exhaust gas treating device which cleans the exhaust gas by using a cleaning solution containing a nanovalve, while this cleaning is not the cleaning of an RO membrane.